Inorganic materials with long-term stability are known per se. Materials which are specifically used as bioactive bone-replacement materials and have a sufficient long-term stability are also described in the relevant literature. For example, there have been numerous publications dedicated to the successful clinical use of glass ceramics and/or sintered glass ceramics with the main crystal phases apatite and wollastonite [Kokubo, T., Biomaterials, 12 (1991) 155-163; Berger, G. et al.: Long-term stable bioactive glass ceramics as an implant material—ten years of clinical experience, Fourth World Biomaterial Congress, Berlin, Apr. 24-28, 1992, Transactions p. 33]. The chemical stability of the aforesaid materials was surpassed by other bioactive materials based on calcium-zirconium/titanium phosphate which can only be produced by means of ceramic methods, but do not form a melt at temperatures which are common in the glass industry (approximately 1,650° C.), which, as is known, brings about disadvantages as regards the mechanical stability of such granulated materials and in particular of molded bodies manufactured therefrom (Biomaterials 18 (1997) 1671-1675).
Furthermore, it is known that metallic implants are roughened on their surface in order to improve bonding to the surrounding tissue. During this blasting operation, particles are always incorporated into the metallic surface owing to tribochemical reactions.
Commonly, commercial methods relate to blasting with Al2O3. The roughening effect on the surface is very good, whereas the concentration of Al2O3 on the surface is problematic from the aspect of biocompatibility. An improvement was achieved through the use of glass-ceramic materials which contain apatite and wollastonite or apatite, wollastonite and Ca7Mg2P2O24 as main crystal phase as described in DE 41 26 800 A1. Here, particles with a thickness of at least 1 μm are incorporated into the surface of the implant at a hardness according to Mohs of 5-7° of the vitreous-crystalline or ceramic materials. The roughness of the surface thus created ranges between 5 and 10 μm.
The chemical long-term stability of the aforesaid materials in connection with their bioactivity (direct connective tissue-free bone contact) has so far been regarded as unsurpassed.
Long-term stable commercial materials are derived from apatites. They are either ceramics with hydroxyapatites or fluoroapatites (HAp, FAp) as main crystal phase and with poor processing properties or they are glass ceramics which in most cases include another crystal phase, e.g. wollastonite for achieving improved mechanical properties, mica phases for achieving machinability, etc. The processing properties of glass ceramics are in general considered to be sufficient. Seen from the aspect of long-term stability, the known combinations of apatites, further crystal phases and/or residual glass components always result in materials whose solubility is higher than that of the pure phases (HAp, FAp).